1. Field of the Invention
This invention relates to a combination convection/microwave oven and, in particular, to a convection/microwave oven with a controller and method for controlling a cook cycle of the oven.
2. Description of the Prior Art
Ovens that use both microwave energy and thermal energy transferred by convection are described in U.S. Pat. Nos. 4,358,653, 4,392,038, 4,410,779 and 4,430,541. For example, U.S. Pat. No. 4,430,541 discloses an oven having a source of microwave energy disposed in a bottom of the oven""s cooking chamber and a blower arranged in a side wall to produce a heated airflow. A food product in a container is situated above the microwave source and in the path of the heated airflow. The container is microwave transparent in order to have as much as possible of the microwave energy directly impinge upon the food product. In ovens of this type, the container is positioned in the microwave energy pattern so that substantially all of the microwave energy is incident on the bottom of the container.
Microwave energy can thaw and cook food products rapidly, but it generally does not provide surface finishing, browning, or other characteristics provided by cooking in an oven environment. Accordingly, microwave ovens with added thermal convection energy have become popular in the restaurant industry. When prior art combination convection/microwave ovens have been used to cook frozen food products, such as biscuits, pies and other bakery goods, dark spots and other non-uniformities often form on the food product. Food products with dark spots are unsightly and, therefore, unpalatable to customers.
The dark spots are formed due to non-uniform energy transfer to and within the food product during the cooking process. The temperature of a frozen food product, for example, can be non-uniform due to conditions existing in the freezer, to non-uniformity of the food product itself, to the package that contains the food product and/or to conditions that occur in the oven. When thawing and/or cooking a frozen food product in prior art ovens, the bottom of the product is warmed by the direct impingement of the microwave energy. However, the top and sides of the food product are being warmed by the heated airflow. The frozen food product cools the heated airflow so as to affect the cooking or thawing temperature of the top and sides. This effect is known as the chill factor as it is similar to the wind chill factor produced by wind on a cold day. As the food product continues to thaw and then to cook, the sides and top remain cooler than the bottom and, thus, enhance the formation of the dark spots or other indications of non-uniform cooking.
Additionally, prior art combination convection/microwave ovens require the use of microwave transparent cooking containers, such as those made with ceramic or glass. This reduces the flexibility of means of thermal transfer and may affect the characteristics of the cooked products.
Thus, there is a need for a combination convection/microwave oven that can rapidly thaw, cook and possibly brown food products with increased uniformity of interior and exterior properties.
A combination microwave and convection oven of the invention cooks a food product with microwave energy from a source of microwave energy and by a heated airflow that is produced by a thermal energy source and a blower. A controller operates the microwave energy source, the thermal energy source and the blower to cook the food product in a cook cycle. The controller includes a processor that performs a procedure that turns the microwave energy source, the thermal energy source and the blower on and off during the cook cycle according to either temperature of the heated airflow or time within the cook cycle.
According to an aspect of the invention, the procedure includes a soak interval during which the thermal energy source and/or the blower is turned off and the microwave source is either turned on or off, thereby allowing temperature within the food product to equilibrate. The soak interval can occur at any point in the cook cycle.
According to another aspect of the invention, the food product can be situated in a microwave reflective pan that is held on a rack by a rack suspension system at a height h above the microwave energy source. The height h is selected so that the microwave reflective container is in the near field of the microwave energy, whereby the food product is cooked by reflected microwave energy and convection of the heated airflow. This aspect provides additional cooking flexibility, as there is no requirement for microwave transparent cooking containers.